283

Emanoil Toma

The Use of Current Generators in Electrical Converter Drivesfor Stepper Motors

This paper presents some ways to realize electrical converters for step-per motor drives. The first part analyzes aspects for unipolar steppermotor and use of constant current generators. The second part presentcurrent sources based on peak limiting current trough the inductance ofmotor coil. A complete drive module for bipolar stepper motor was con-ceived and simulation results confirm their functionability.

Keywords: current limiting, micro-stepping, Pspice simulation.

1. Introduction

A stepper motor can be considered a digital electromechanical system whereineach electrical pulse input moves the rotor by a discrete angle called the stepangle and hence the name stepping motor.

Stepper motors are used in commercial and industrial applications because oftheir high reliability, low cost, high torque at low speeds and a simple constructionthat operates in almost every environment [2].

Using a stepper motor is possible to design applications in simple open-loopcontrol system. In this case mechanical actuated system, have low accelerationwith static loads.

Stepper motor haves the advantage of producing high torque at low speed.Stepper motor haves holding torque which allows keeping its position firmly whennot turning. Holding its position is useful for intended application where the motorwill be started and stopped. In this time the force of the mirror’s momentum actingagainst the motor remains present. The results will be unnecessary of brake mec-hanism.

A stepper motor requires a signal generator and a pulse distributor power sta-tic switching like is showed in figure 1.

The forcing of current through stepper’s phases can be realized in many ways:Series resistance, resistors and capacitors, constant current electronic generator,two power supply voltage source and “chopper” method [1], [5].

ANALELE UNIVERSITĂŢII

“EFTIMIE MURGU” REŞIŢA

ANUL XXI, NR. 2, 2014, ISSN 1453 - 7397

284

TranslatorSwitch

Set

Stepper

Motor

Mechanical

Torque

Step

Direction

Figure 1. Stepper motor drive elements

2. Simulations for unipolar stepper motor drivers

For experimental set a so called "eight wires" hybrid stepper motor was used.This can be configured as unipolar or bipolar mode connection (figure 2)

1

23

4

5 6 7 8

Unipolar connection

1

23

4

5 6 7 8

Bipolar series connection

1

23

4

5 6 7 8

Bipolar parallel connection

A

C

B

D

com

A

D

B

C

A

D

B

C

Figure 2. Connection of "8 wires" Stepper Motor

A' C'

D1 D2 D3 D4

SW1 SW2 SW3 SW4Valimentare

1

23

4

5 6 7 8

Unipolar connection

C'

A'

com

D'

B'

B' D' com

Figure 3. Unipolar connection driver

The states of motor coils and chronograms of current for full step - doublecommand are showed in figure 4, where an exaggerated 45o angle for a full step isfor evidence of phenomenon. The usual commercial hybrid motors have 1.8o stepangle. Red color for wire suggest the north magnetic pole, blue color suggest thesouth magnetic pole, and black color suggest an inactivated coil.

285

C

B

A

D

321 214 43 1 3 2 214 4 3 1

Right direction (CW) Left direction (CCW)

Figure 4. States of motor's coils and current evolution

Electrical parameters of motor coil are: resistance R=1.1Ω, inductanceL=1.7mH, nominal current Imax=3A, and these values was used in the next simula-tion examples.

A time constant for motor's coils determines a slow increase and decrease ofcurrent trough the motor's coil. Rise time and fall time are approximately fourtimes constant of motor's coil.

msH

R

L 54.11.1107.1 3

, (1)

Rise time and fall time of current trough the motor's coil can be reduced byadding an external resistor in series with motor coil and implicit by using a relative-ly high value of voltage source In simulation schemas used in figure 5, a resistor of2.9 Ω was introduced for limiting current at 3A for a voltage source of 12V.

msH

RR

L

ext

4.0)9.21.1(

107.1 3

, (2)

286

0

VCDTD = 0

TF = 1nPW = 20msPER = 40ms

V1 = 0

TR = 1n

V2 = 12V

L11.7mH

R11.1

V1

3.3V

M1

IRFZ44

00

D1MUR405

L21.7mH

R21.1

V2

12V

0

M2

IRFZ44

0

D2MUR405

CD

R_EXT2.9

Motor MotorCoil Coil

Switch Switch

II

V

V

Figure 5. Pspice Simulation Schemas for one motor coilwithout and with external resistor

Ti me

0s 5ms 10ms 15ms 20ms 25ms 30ms 35ms 40ms 45ms 50ms 55ms 60ms 65ms 70msI ( L2) I ( L1)

-1. 0A

0A

1. 0A

2. 0A

3. 0A

SEL>>

wi t hout _R_EXT wi t h_R_EXT

V( CD)0V

4V

8V

12V

Figure 6. Simulation results for the anterior schemas

287

A voltage source with a series resistance begins to have a current source be-havior. Replacing the external resistor with a current source, power dissipated onhim can be reduced [1]. For unipolar motor with four phases is necessary only twoconstant current sources as shown in figure 7.

C'A'

D1 D3 D4

SW1 SW3 SW4V_low

1

23

4

5 6 7 8

Unipolar connection

com_1

A'

C'

B'

D'

D'

com_2

D2

SW2

B'

V_high

I1

I2

I1=I2=Inom

Figure 7. Constant current sources used for unipolar connection

Implementation of constant current source can be realized with operationalamplifier and power transistor like in figure 8, where V_REF can be obtained usinga reference voltage integrated circuit and a voltage divider.

0

V_HIGH

12V M1

IRFZ44

M3

IRFZ44

L1

1.7mHL3

1.7mH

R1

1.1

R3

1.1

0 0

+

- V+

V-

OUT

U1A

LF442/ON

D2

MUR810

D3MUR810

D1

MUR810

C1220N

0 0

COM1

0

V_REF0.3V

0

V_LOW

8V

A C

V1TD = 0

TF = 1nsPW = 20msPER = 40ms

V1 = 0

TR = 1ns

V2 = 12V

0

G1V3

TD = 0

TF = 1nsPW = 20msPER = 40ms

V1 = 12v

TR = 1ns

V2 = 0

0

G3

M4

IRF9540

R4

0.1

VV

V

VV

II

Figure 8. Electrical schema used for Pspice simulation of unipolar stepper motordrive using a constant current source (one pair of motor coils).

288

For the results of simulation, presented in figure 9, we can observe presenceof voltage oscillations on drain terminal for MOSFET in OFF-state and also in com-mon point of coils pair (V(COM1)). A capacitor C1 can reduces this oscillations, buta high value determines an oscillation component on coil's current. Unfortunatelythe value of voltage source (V_LOW) used for constant current generator must beabove of middle of the source used for rapidly decrease of current (V_HIGH). Bythe way the electrical efficiency is poor.

Ti me

0s 5ms 10ms 15ms 20ms 25ms 30ms 35ms 40ms 45ms 50msV( COM1)

-10V

0V

10VV( C) V( A)

-20V

0V

20VI ( L3) I ( L1)

-4. 0A

0A

4. 0A

SEL>>

V( G1) V( G3)0V

10V

20V

Figure 9. Pspice simulation results for anterior electrical schema

3. Simulations for bipolar stepper motor drivers

Two H-Bridges are necessary for our bipolar stepper motor, one for each mo-tor phases.

D5

D6

D7

D8

SW5

SW6

SW7

SW8

A BC D

D1

D2

D3

D4

SW1

SW2

SW3

SW4

Valimentare

1

23

4

5 6 7 8

Parallel bipolar conection

C

A

D

B

Figure 10. Bipolar connection driver

289

Parallel connection assures the great couple. The coil's resistance becomes0.55 Ω and inductance remains 1.7 mH. The bridge connection assures energy re-covering when the current decreases. For full step - double command the currentflow permanently trough the each coils. The limited value for our motor is neces-sary to be 4 A, not 2x3 amperes. The states of motor coils and current's chrono-grams are showed in figure 11. Red color for wire suggest the north magnetic poleand blue color suggest the south magnetic pole created by active coils.

1

B-D

A-C

432 21 143 24 3 3 21 14

Left direction (CCW)Right direction (CW)

Figure 11. States of motor's coils and current evolution

+

UH

-

0

0

.R

R

S

S

0

+

-

OUT

ComparatorU_REF

RS Latche

Current Sense

u=i

L R

D1

D2

D3

D4

SW1

SW2

SW3

SW4

Motor CoilDriver

Driver

Driver

Driver

S Q

R Q

PWM Clock

Figure 12. Peak current limiting principia schema for one phase of bipolar motor

290

In figure 12 is showed the principia schema to limit current trough the motorcoil, using logic and analog circuits. The current sense resistor gives a voltage pro-portional with the current trough the motor coil. This value is compared with thereference voltage. When the current exceeds a maximum imposed value, the out-put of analog comparator resets Latch, the output Q becomes zero logic, the PWMpulse is finished and switches SW1, SW4 is turned "OFF" and switches SW2, SW3is turned "ON", current trough the motor coil becomes decrease. In the next ex-amples, detailed simulation schemas was conceived based on figure's 13 diagram.

H-BRIDGE

A

B

H-BRIDGE

G5MOSFET

G6

G7

G8

LIMITING

&

PULSES

DISTRIBUTION

PHASES 1

23

4

5 6 7 8

A

B'

A'

C'

C

B DD'PWM Clock CLOCK_PWM

GENERATOR

PAS A

SENS B

PEAK-CURRENT

REF_A

CRT_A

REF_B

CRT_B

BLOCARE_PUNTE

CD_1A

Drivers_A

CD_2A

CD_1B

Drivers_B

CD_2B

G1MOSFET

G2

G3

G4

Figure 13. Block diagram of conceived bipolar motor driver module.

VDD

SE

T8

D9

CLK11

Q13

Q12

VD

D14

VS

S7

RE

SE

T10

U2B

CD4013B

0

RESET

0

VDD

R810k

C210n

IC = 0

VDD

0

A

147

11

13

12

U8D

CD4077B

0

0

V

SENS

V7

TD = 0

TF = 1nPW = 31.5msPER = 62.5ms

V1 = 0

TR = 1n

V2 = 12

0

VDD

0

V6

TD = 0

TF = 1nPW = 1msPER = 2ms

V1 = 0

TR = 1n

V2 = 12

SE

T6

D5

CLK3

Q1

Q2

VD

D14

VS

S7

RE

SE

T4

U5A

CD4013B

0

VDD

PAS

0

B

147

3

1

2

U10A

CD4077B

0RESET

VDD

SE

T8

D9

CLK11

Q13

Q12

VD

D14

VS

S7

RE

SE

T10

U13B

CD4013B

0

RESET0

Figure 14. Detailed electrical schema for simulation of phases generator

291

In figure 13 we can observe the bipolar parallel configuration of our "eightwires" stepper motor with H-Bridge A and drivers A for one motor's coil and H-Bridge B and drivers B for another motor's coil. Five input-signals are necessary forour module: PAS is the signal for step, active on the rising slope. SENS is the signalfor direction of rotation, level "0" for clockwise (CW) direction and level "1" forcounterclockwise (CCW) direction. BLOCARE_PUNTE is signal for standby, the cur-rent trough the motor is cut-off when this signal is on level "0". The two signalsREF_A and REF_B, are the voltage references for peak current limiting techniquedescribed in figure 12. They are positive constant voltages for full-step double-command. These signals must be correlated with PAS signal when micro-steppingcommand is used. They can be easily obtained using a microcontroller and twodigital to analogue converters (DAC) [3, 4].

For simulation, two phase shifted sinusoidal signal and two precise rectifierswas used (figure 17).

Phases generator block, detailed in figure 14, creates signals for the twophases A and B, based on PAS and SENS signals. For CW direction the phase B isbehind the phase A and for CCW direction the phase B is before the phase A.

Peak-current limiting and pulses distribution module, figure 15, assures thecomplementary inputs signals for each H-bridge's driver.

0

BLOCARE_PUNTE

0

0

VDD

VDD

VDD

VDD

1

2

147

3

U15A

CD4081B

5

6

147

4

U16B

CD4081B

8

9

147

10

U16C

CD4081B

13

12

147

11

U16D

CD4081B

V8

TD = 42.5m

TF = 1nPW = 20msPER = 113.5ms

V1 = 12

TR = 1n

V2 = 0

0

CD_1A

CD_2A

CD_2B

CD_1B

V

V

V

V

V

0

R7

1k

A

C110nIC = 0

0

comp_ACRT_A

REF_A

OUT7

+2

-3

G1

V+

8V

-4

B/S6B

5U4

LM311

VDD

VDD

0

CLOCK_PWM

VEE

0

R61k

V5

TD = 0

TF = 1nPW = 0.2usPER = 50us

V1 = 0

TR = 1n

V2 = 12

SE

T6

D5

CLK3

Q1

Q2

VD

D14

VS

S7

RE

SE

T4

U9A

CD4013B

VDD

RESET_A

0

0

VDD

147

4

56

U17B

CD4077B

0

B

R10

1k

C310nIC = 0

0

comp_BCRT_B

REF_B

OUT7

+2

-3

G1

V+

8V

-4

B/S6B

5

U6

LM311

VDD

VDD

0

CLOCK_PWM

VEE

VDD

R111k

147

10

89

U17C

CD4077B

0

SE

T6

D5

CLK3

Q1

Q2

VD

D14

VS

S7

RE

SE

T4

U11A

CD4013B

RESET_B

VDD0

0

VDD

147

4

56

U12B

CD4077B

0

VDD

147

10

89

U14C

CD4077B

0

Figure 15. Detailed electrical schema used for simulation of peak-current limitingand pulses distribution module

292

In simulation example, H-bridges was realized with MOS complementary tran-sistors. This can be an optimal choice if power supply is 12V, digital and analogintegrated circuits is biasing by the same power supply, a zero volts level for gateassures "ON" state for P-MOS and "OFF" state for N-MOS. A 12 volts level for gateassures "OFF" state for P-MOS and "ON" state for N-MOS. Bipolar transistors classof B amplifier is a voltage follower and current amplifier and assures rapidly chargeand discharge of input capacitor of Power-MOS transistor. For higher current capa-bility, only N-MOS transistors is recommended and adequate drivers based onbootstrap technique. In this case power supply voltage can be higher than 12V.

CD_1A CD_2A

I

Q5

BC547B

Q6

BC557B

VDD

G4

0

L3 1.7mH

IC = 0

R3

0.55

DMBR845/OND1

0

R50.1

0

BRAT_2ABRAT_1A

G4

CRT_A

G2 G3

G1

VDD

VEE

V2

5V

0

Q1

BC547B

Q2

BC557B

VDD

0

G3

Q3

BC547B

VDD

Q4

BC557B

0

G1V1

12V

0

M1

IRF9540

M4

IRF9540

M2

IRF540

M3

IRF540

Q7

BC547B

VDD

Q8

BC557B

G2

0

Figure 16. H-Bridge and his drivers used for simulation

V10

FREQ = 125VAMPL = 0.37VOFF = 0

PHASE = 90

0

D11

D1N4148

D10

D1N4148 D13

D1N4148

D14

D1N4148 0

R19 20k

R20 10k

R21 20k

VEE

VDD

C23 100n

+3

-2

V+

8V

-4

OUT1

U19A

TL082

+5

-6

V+

8V

-4

OUT7

U19B

VDD

VEE

R22 20k

R23 20k

0

0C24 100n

0

REF_A

00 0

D12

D1N4148

D15

D1N4148

Figure 17. Precise rectifier used for simulation

293

The results of simulation for conceived electrical schema is presented in figure18, where we can observe the sifting of phase B behind the phase A for logic level"1" of SENS signal and before for logic level "0". Sinusoidal waves for currenttrough motor's coils was obtained. Frequency of sinusoidal motor's current is 125Hz for the simulated example. Period of this wave is 8 ms. Four completely steps isexecuted during a sinus period. That implies a 2 ms duration for one step. A com-pletely revolution have 200 steps. Therefore a revolution duration is 400 ms. Thatimplies a speed of 150 RPM. For increase the motor speed is necessary to increasefrequency of reference voltage signals correlated with step signal. But for high fre-quency of sinusoidal current the inductive component of motor voltage becomespredominant.

)( motormotormotormotor LjRIU , (3)

For a speed of 140 RPM the inductive component of motor voltage is:

VLfIU 125.5107.1120242 3 , (4)

For 300 RPM, the inductive voltage component becomes 10.25 volts. Forhigher speed is necessary a higher voltage power supply and preferable N-MOStransistors H-bridges and drivers based on bootstrap technique.

Ti me

80ms 85ms 90ms 95ms 100ms 105ms 110ms 115ms 120ms 125ms 130ms 135ms 140msV( SENS)

0V

10V

20V

SEL>>

80ms 85ms 90ms 95ms 100ms 105ms 110ms 115ms 120ms 125ms 130ms 135ms 140ms

A B

I ( L4)-4. 0A

0A

4. 0AV( REF_A) V( REF_B)

-400mV

0V

400mVI ( L3)

-4. 0A

0A

4. 0A

Figure 18. Simulation results for current evolution trough the stepper motor'scoils for bipolar connection and rectified sinusoidal wave for reference voltage.

294

4. Conclusion

Using a stepper motor is possible to design applications in simple open-loopcontrol system. Stepper motor haves the advantage of producing high torque atlow speed. The time-constant of motor coil determines a slow increase and de-crease of current trough the motor's coil. That limits the maximum speed of motor.For increasing the maximum speed is necessary an external resistor, but electricalefficiency is poor. Using constant current sources the electrical efficiency increase.Linear current sources however have a small electrical efficiency.

Current sources based on chopped technique considerably improving the elec-trical efficiency. For the small motors drives there are many integrated circuits. Forbig motors drives are necessary to use discrete power MOS transistors. A completedrive module for bipolar stepper motor was conceived and simulation results con-firm their functionability. Also this module permits the control of sinusoidal currenttrhough the motor coils.

References

[1] Acarnley P.P., Stepping motors: a guide to modern theory and practice,4-th Edition, The Institution of Engineering and Technology, 2002.

[2] Aranjo B., Soori P.K., Talukder P., Stepper motor drives for robotic ap-plications , Power Engineering and Optimization Conference (PEDCO), 6-7June, Melaka, Malaysia, 2012, 361-366.

[3] Baluta G., Microstepping Mode for Stepper Motor Control, InternationalSymposium on Signals, Circuits and Systems, 12-13 July, Iasi, Romania,2007, Vol. 2, 1-4.

[4] Baluta G., Coteata M., Precision Microstepping System for Bipolar Step-per Motor Control, International Aegean Conference on Electrical Ma-chines and Power Electronics, 10-12 September, Bodrum, Turkey, 2007,259-264.

[5] Toma E., Simion C.M., Bogdan L., Improving of Stepper Motor Beha-viors Used on Cinematic Chain of Machine-Tools for Electrical DischargeMachining, Nonconventional Technologies Review, Volume XVII, no. 2,2013, 97-102.

Address:

As. Drd. Eng. Emanoil Toma, “Lucian Blaga” University of Sibiu, Str.Emil Cioran, nr. 4, 550024, Sibiu, emanoil.toma@ulbsibiu.ro